Mixing and agitating devices are in wide use in industry. For example, mixing devices are well known in the industrial, pharmaceutical, biotechnology, and other materials processing industries. In one common type of mixer, a vessel contains the material to be mixed along with a rotating shaft that has one or more sets of radially extending impellers that extend radially from the shaft. Typically, a number of impellers are evenly circumferentially spaced around a central hub. The impellers may have for example a substantially flat blade profile shape, a curved blade profile shape, or an air foil type of blade profile shape, depending on the mixing application and the design considerations of the mixing device.
In the case of flat blades or simple curved or bent blades, it has been relatively convenient to manufacture these blades by taking metal sheet stock and bending it if necessary and welding the flat, bent or curved piece, made from a single sheet, to the hub.
In the case of blades having more complex or compound curved shapes, particularly in three dimensions, such as is needed for air foil (or wing) shaped blades, however, the construction process is more complex. In one type of prior art air foil shaped impeller blade, the blade has been constructed as follows. First, an internal skeleton has been constructed having a generally lattice type framework which has as a part of the framework a first piece of elongated bar stock that will become the leading edge of the blade and a second piece of elongated shaped stock that will become the trailing edge of the blade. The first bar stock has typically been circular in cross section. The second bar stock typically has a custom tapered shape in cross section.
A top skin and a bottom skin are then mounted over the lattice, with the front edge of the top skin being welded to an upper surface of the front bar stock, and the front edge of the bottom skin being welded to a lower edge of the front bar stock. Thus a leading edge of the blade is provided. In order provide smoother flow at the leading edge region, after the welds are made they are ground down smooth to form a smooth connection between the bar stock and the top and bottom plates respectively.
At the rear, or trailing, edge of the above described design, the top plate has also been welded to an upper surface of the trailing edge and the bottom plate has been welded to a lower surface of the trailing edge. Since it is desired for the trailing edge to have a relatively pointy taper, the trailing edge has been a relatively difficult piece to shape, and has generally been a custom machined part. The welds where the top skin meet the trailing edge piece and where the bottom skin meets the trailing edge piece have been generally ground down smooth to form a smooth contour between the top and bottom skins respectively and their connection at upper and lower portions of the machined trailing edge piece.
Further in the above described design, a tip piece has been mounted at the radial outside edge of the wing shaped impeller. This tip is required to have a relatively complex compound shape, since it needs to follow the air foil side profile when view from the end, and also generally has a rounded outer surface when seen in plan view. In the prior art, the tip was generally made of an oversized and somewhat blocked shaped piece that was then contoured in all three axes by labor intensive hand grinding to fit the desired 3-D profile.
The above described construction method, while providing satisfactory impellers, does suffer from some disadvantages. First, a total of four welds are required at the leading and trailing edges (that is, two welds at the leading edge and two welds at the trailing edge). Also, a piece of front bar stock and a piece of the rear bar stock are required. In addition, an interior skeleton is required for sparring between the front and rear bar stocks and locating them relative to each other during the assembly process. Due to the weight added by the skeleton, the overall weight of the finished impeller is thus increased for a given skin thickness. Impeller blades moving through material are subject erosion over time. The effective of erosion can be particularly pronounced with mixing abrasive material such as for example materials containing aluminum. Erosion is particularly undesirable at the welded area, because where the bar stock meets the weld, which in turn meets the top or bottom skin, the materials may wear at different rates causing roughness or discontinuity in the flow at that location, which further exacerbates the erosion problem at that location, leading to greater discontinuity and more erosion and so on.
In addition, high wear areas typically occur on the top surface of the air foil, and as a result the welds that are attaching the top skin tend to be the first to wear out since they are exposed to the top fluid path surface. Furthermore, the process of shaping the end tip after it has been welded onto the blade is somewhat labor intensive and is complicated by the fact that the shaping is not performed until the tip has been welded onto the end of the blade, instead of at a possibly more convenient time and location in the overall blade manufacturing process.
In view of the foregoing, it would be desirable to have an improved impeller blade structure and method that can alleviate the above described difficulties at least to some extent.